Field of the Invention
This invention relates to a laser light emitting device operating as a light source for the laser light which for which a moderately wide spectral width is required.
Heretofore, attempts have been made for utilizing a high output laser in a variety of industrial fields by exploiting monochromaticity (narrow spectral band) of the laser light. In particular, the laser light excited by a laser light source employing neodymium aluminum garnet (Nd:YAG) oscillating at a longitudinal single mode by a Q-switching method, referred to hereinafter simply as a Nd:YAG switch laser, and the laser light obtained on wavelength conversion of this laser light, exhibit high peak intensity, so that such laser light is expected to be used in a number of industrial fields.
Such Q-switch laser oscillates in general in longitudinal multiple mode oscillation. If the line width (spectra width) of the frequency components is broader, the problem of chromatic aberration is raised. Although it is attempted to excite the laser light in a sole wavelength using an injection seed technique, the oscillation spectral width becomes excessively narrow to produce inconveniences in utilization.
The laser light having high monochromaticity, that is with a narrow spectral width, has high coherence and is susceptible to noise produced by the interference pattern (speckle noise) caused by interference of the laser light itself with irregular phase relation with diffused light, such as stray light having different propagation distances. Conversely, the laser light with low monochromaticity, exhibits low coherence, however, it has a broad spectral width and is susceptible to chromatic aberration.
The laser light is used in, for example, a laser beacon device. This laser beacon device is investigated as means for improving resolution of an optical system employed in observation of heavenly bodies or in intersatellite light communication, as described in "Laser Beacon Adaptive Optics", Physics news, pp.14 to Jun. 19, 1993.
The laser beacon device radiates a laser light into air and emits light from sodium atoms in atmosphere by resonant absorption. The laser beacon device operates for detecting atmospheric disturbances by detecting the light emitted by the sodium atoms on the ground surface, while operating for correcting the atmospheric disturbances using an adaptive optical system for improving resolution of a telescope.
The sodium atoms emit light on resonant absorption of the laser light of a wavelength in the vicinity of 589 nm. For realizing high efficiency resonant excitation of sodium atoms, a high output laser light source, correctly coincident with the absorption spectrum of sodium atoms in both the frequency and frequency width, is required.
As a high-output light source having a wavelength of 589 nm, there has so far been proposed a laser light source comprised of a first laser light source 101 having a Nd:YAGQ switch laser generating a fundamental wavelength laser light of a wavelength of 1319 nm narrowed in frequency using an injection seed technique, a second laser light source 102 having a Nd:YAGQ switch laser generating a fundamental wavelength laser light of a wavelength of 1064 nm similarly narrowed in frequency and an LBO crystal 103 for generating an additive frequency laser light from the fundamental wavelength laser light with the wavelength of 1319 nm and the fundamental wavelength laser light with the wavelength of 1064 nm.
The injection seed technique, used in the first laser light source 101 and in the second laser light source 102, excites the laser light by oscillation in a sole wavelength for correctly coinciding the wavelength of the laser light after additive frequency generation with the absorption wavelength of sodium atoms.
The absorption frequency width of sodium atoms, subjected to the Doppler effect, is on the order of 3 GHz. Since the laser light with the wavelength of 589 nm, obtained by a high output light source shown in FIG. 1, is narrowed in frequency by injection seed in each light source, the laser light obtained by the high output light source exhibits high frequency stability. The spectral width of the basic wavelength laser light, obtained by the high output laser light source, is narrowed up to approximately 25 MHz, which is the line width of a transform limited pulse, and which is only 1/120 of the absorption frequency width of sodium atoms. Therefore, the resonance efficiency between the laser light and the sodium atoms is low.
The laser light is also applied to, for example, a laser image display device.
In the above laser image display device, if a laser light beam with a narrow spectral width is used, the speckle noise tends to be produced because of the high coherence of such laser light. Such speckle noise generates granular speckles in the laser image display device, thus significantly deteriorating the picture quality.
Thus, in the application of the laser light, it is crucial that the spectral width of the laser light be controlled to a moderate value for obviating problems produced in connection with chromatic aberration and speckled noise.
Several methods for broadening the spectral width of the laser light has so far been proposed. For example, it may be envisaged to oscillate the laser light in longitudinal multiple mode or to use the laser light oscillated in longitudinal multiple mode from the outset. In this case, the spectral width tends to be broadened excessively to raise the problem in connection with chromatic aberration. In addition, the structure of the laser light generating device itself needs to be changed, thus lowering the light emitting efficiency. That is, it has been difficult to increase the spectral width moderately to a desired value.
Furthermore, the laser light beam obtained on wavelength conversion on additive frequency mixing of two or more sorts of laser light beams oscillated in longitudinal multiple modes is unstable in intensity.
For removing the speckle noise, attempts have been made in improving the laser light projection system. For example, it has been proposed in JP Patent Kokai Publication JP-A-55-65940 (1980) that, in a laser image display device, a screen or a laser light source is oscillated mechanically. This, however, is infeasible if the screen size is larger.
Thus it is difficult to control the spectral width of the laser light to a desirable value to remove the speckled noise.